Transmission apparatus, reception apparatus, transmission method, reception method, and method for generating multi-dimensional constellations

ABSTRACT

The present invention relates to digital data communication and provides an efficient method for generating multi-dimensional constellations for digital data modulation with a high degree of modulation diversity, a method for transmitting and receiving data on the basis of such constellations, and a corresponding apparatus. This is achieved by considering only multi-dimensional rotation matrices with all elements on the diagonal having the same first absolute value and all other elements having the same second absolute value. In this manner, multi-dimensional rotation matrices can be generated having only a single independent parameter and a structure that is as regular as possible. The independent parameter can be configured in order to minimize the error probability for various constellation sizes.

TECHNICAL FIELD

The present invention relates to digital data communication, inparticular to methods for generating multi-dimensional constellationsfor digital data modulation, methods for modulating and transmittingdata on the basis of multi-dimensional constellations, and acorresponding apparatus.

BACKGROUND ART

Fading is one of the major problems in communication systems. Itrepresents random fluctuations in the amplitude of the received signaldue to multi-path propagation. If the delay spread of the channel islarger than the symbol period of the signal, the fading is alsofrequency selective. The amplitude of fading is usually approximated bya Rayleigh distribution. Such fading is referred to as Rayleigh fading.

In digital communication systems, information is encoded as a sequenceof symbols belonging to a discrete alphabet, referred to as aconstellation. Such a constellation has N dimensions and encodes Binformation bits per dimension. The number of possible values, alsoreferred to as constellation points, is therefore 2^(N*B). The number ofbits per dimension B directly determines the spectral efficiency of thetransmission, given in bits/Hz. The number of dimensions N has no effecton the spectral efficiency. An example constellation with N=2 and B=1 isillustrated in FIG. 1A.

Traditionally, for example in a quadrature amplitude modulation (QAM)constellation shown in FIG. 1A, each transmitted bit affects only onedimension. Referring to FIG. 1A, “b₁” of each constellation point “b₁b₂”(=“00”, “01”, “10” and “11”) affects only the dimension represented bythe horizontal axis, whereas “b₂” of each constellation point “b₁b₂”affects only the dimension represented by the vertical axis. If thedimension affected by the transmitted bits undergoes a deep fading, allbits that modulate this dimension will be extremely unreliable, whichincreases the error probability. This effect is illustrated by theerrors in FIG. 1A. For example, if the channel represented by thevertical axis fades away, the constellation points “00”, “01”, “10” and“11” will approach the horizontal axis (along the solid arrows of FIG.1A). As a result, the constellation points “00” and “01”, as well as theconstellation points “10” and “11”, will be indiscernible.

If the constellation is modified such that each bit affects alldimensions, the resilience to fading is increased. A deep fading on oneof the dimensions will affect all the bits of the constellation;however, this effect would not be as detrimental as in the conventionalcase, so that on average, the error probability decreases. This isreferred to in the literature as modulation diversity.

(Rotated Constellations)

One way to achieve modulation diversity is to rotate a (hyper-cubic)constellation to spread the effect of a channel fading over all itsdimensions. This is illustrated in FIG. 1B for the case where N=2 andB=1. For example, as shown in FIG. 1B, if the channel represented by thevertical axis fades away, the constellation points “00”, “01”, “10” and“11” will approach the horizontal axis (along the solid arrows of FIG.1B). However, these constellation points will still be discernible inthe dimension represented by the horizontal axis. As such, theconstellation points “00”, “01”, “10” and “11” remain discernible evenafter a deep fading of the channel represented by the vertical axis.

A multi-dimensional rotation can be achieved by multiplying theN-element signal vector by an N*N square matrix. The necessary andsufficient condition for a square matrix to be a rotation matrix (or areflection matrix) is for it to be orthogonal, i.e., to satisfy theequation of the following Math. 1.

^(T) =I  [Math. 1]

Note that in the above Math. 1, the matrix

-       is a square matrix, the matrix-   ^(T)    is a transpose matrix of the matrix-   ,    and the matrix-       is a unit matrix.

This means that with regard to the above Math. 1, the row/column vectorsmust be orthogonal unit vectors, i.e., satisfy the equation of thefollowing Math. 2.

$\begin{matrix}{{\sum\limits_{i = 1}^{n}\;{r_{i,j}r_{i,k}}} = \delta_{j,k}} & \left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

Note that in Math. 2,

-   -   δ_(j,k)=1        if    -   j=k ,        and    -   δ_(j,k)=0        if    -   j≠k.

This preserves the Euclidean distance between any two points of theconstellation, and ensures that the performance in channels withadditive white Gaussian noise (AWGN channels) is not affected.

Obviously, not all rotations yield the effect of improved modulationdiversity. From NPL 1, it is known that the optimum rotation angle

-   θ    for 16-QAM satisfies the equation shown in the following Math. 3.    The corresponding 2-D (two-dimensional) rotation matrix-       satisfies the equation shown in the following Math. 4.

$\begin{matrix}{\theta = {\pi/8}} & \left\lbrack {{Math}.\mspace{14mu} 3} \right\rbrack \\{R = \begin{pmatrix}{\cos\;\theta} & {{- \sin}\;\theta} \\{\sin\;\theta} & {\cos\;\theta}\end{pmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 4} \right\rbrack\end{matrix}$

Finding the optimum rotation for constellations of more than twodimensions is more complicated, because there is no single optimizationparameter such as the one pertaining to the rotation angle in a 2-Dconstellation. In the case of a 4-D (four-dimensional) constellation,for example, there are six independent rotation angles, each with itsown partial rotation matrix. The partial rotation angles are also calledGivens angles in NPL 2. The final 4-D rotation matrix is obtained bymultiplying the six Givens rotation matrices, namely the six matricesshown in the following Math. 5.

$\begin{matrix}{{R_{4}^{1,2} = \begin{pmatrix}{{+ \cos}\;\theta_{1,2}} & {{- \sin}\;\theta_{1,2}} & 0 & 0 \\{{+ \sin}\;\theta_{1,2}} & {{+ \cos}\;\theta_{1,2}} & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{pmatrix}},{R_{4}^{1,3} = \begin{pmatrix}{{+ \cos}\;\theta_{1,3}} & 0 & {{- \sin}\;\theta_{1,3}} & 0 \\0 & 1 & 0 & 0 \\{{+ \sin}\;\theta_{1,3}} & 0 & {{+ \cos}\;\theta_{1,3}} & 0 \\0 & 0 & 0 & 1\end{pmatrix}},{R_{4}^{1,4} = \begin{pmatrix}{{+ \cos}\;\theta_{1,4}} & 0 & 0 & {{- \sin}\;\theta_{1,4}} \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 0 \\{{+ \sin}\;\theta_{1,4}} & 0 & 0 & {{+ \cos}\;\theta_{1,4}}\end{pmatrix}},{R_{4}^{2,3} = \begin{pmatrix}1 & 0 & 0 & 0 \\0 & {{+ \cos}\;\theta_{2,3}} & {{- \sin}\;\theta_{2,3}} & 0 \\0 & {{+ \sin}\;\theta_{2,3}} & {{+ \cos}\;\theta_{2,3}} & 0 \\0 & 0 & 0 & 1\end{pmatrix}},{R_{4}^{2,4} = \begin{pmatrix}1 & 0 & 0 & 0 \\0 & {{+ \cos}\;\theta_{2,4}} & 0 & {{- \sin}\;\theta_{2,4}} \\0 & 0 & 1 & 0 \\0 & {{+ \sin}\;\theta_{2,4}} & 0 & {{+ \cos}\;\theta_{2,4}}\end{pmatrix}},{R_{4}^{3,4} = \begin{pmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & {{+ \cos}\;\theta_{3,4}} & {{- \sin}\;\theta_{3,4}} \\0 & 0 & {{+ \sin}\;\theta_{3,4}} & {{+ \cos}\;\theta_{3,4}}\end{pmatrix}}} & \left\lbrack {{Math}.\mspace{14mu} 5} \right\rbrack\end{matrix}$

From NPL 2, it is known that the optimization may be carried out overthe vector having the six elements shown in the following Math. 6.θ=(θ_(1,2),θ_(1,3),θ_(1,4),θ_(2,3),θ_(2,4),θ_(3,4))  [Math. 6]

According to NPL 2, the resulting optimum rotation angles for a 4-Dconstellation with two bits per dimension have the values shown in thefollowing Math. 7.

$\quad\begin{matrix}\left\{ \begin{matrix}{\theta_{1,2} = {39{^\circ}}} \\{\theta_{1,3} = {25{^\circ}}} \\{\theta_{1,4} = {43{^\circ}}} \\{\theta_{2,3} = {53{^\circ}}} \\{\theta_{2,4} = {41{^\circ}}} \\{\theta_{3,4} = {23{^\circ}}}\end{matrix} \right. & \left\lbrack {{Math}.\mspace{14mu} 7} \right\rbrack\end{matrix}$

The disadvantage of this method is the number of parameters,specifically for a large number of dimensions. For N dimensions, thenumber of partial rotation angles is equal to the number of possiblecombinations of two from a set of N, i.e., the value given by thefollowing Math. 8.

$\begin{matrix}{\begin{pmatrix}N \\2\end{pmatrix} = {\frac{N!}{{2!}{\left( {N - 2} \right)!}} = \frac{N\left( {N - 1} \right)}{2}}} & \left\lbrack {{Math}.\mspace{14mu} 8} \right\rbrack\end{matrix}$

Hence, the number of rotation angles increases with the square of thenumber of dimensions, so the optimization problem becomes very difficultwhen the number of dimensions is large.

NPL 3 discloses two different approaches, relying on the use of thealgebraic number theory, which have the advantage of a reduced number ofparameters.

The first approach allows the construction of rotation matrices byapplying the “canonical embedding” to an algebraic number field. Twomethods are proposed. The first method produces lattices with diversityL=N/2 for the number of dimensions N=2^(e2)3^(e3), with e2, e3=0, 1, 2,. . . . Diversity means the minimum number of different values in thecomponents of any two distinct points of the constellation. The secondmethod produces lattices with diversity L=N. The possible values of Nare very limited, such as 3, 5, 9, 11, and 15.

A variant of this method for generating N-dimensional rotatedconstellations is also known from NPL 3. The rotation matrix

-       is expressed by the following Math. 9.

$\begin{matrix}{R = {\sqrt{\frac{2}{N}}{\cos\left( {{\frac{2\;\pi}{8\; n}\left\lbrack {{4 \times \left\lbrack {1,2,\ldots\mspace{14mu},N} \right\rbrack} - 1} \right\rbrack}^{T}\left\lbrack {{2 \times \left\lbrack {1,2,\ldots\mspace{14mu},N} \right\rbrack} - 1} \right\rbrack} \right)}}} & \left\lbrack {{Math}.\mspace{14mu} 9} \right\rbrack\end{matrix}$

Note that the superscripted letter “^(T)” denotes the transpose of amatrix.

For N=4, the value of the rotation matrix

-       is given by the following Math. 10.

$\begin{matrix}{R = \begin{pmatrix}{+ 0.5879} & {- 0.1379} & {- 0.6935} & {- 0.3928} \\{+ 0.1379} & {- 0.3928} & {+ 0.5879} & {- 0.6935} \\{- 0.3928} & {+ 0.6935} & {- 0.1379} & {- 0.5879} \\{- 0.6935} & {- 0.5879} & {- 0.3928} & {- 0.1379}\end{pmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 10} \right\rbrack\end{matrix}$

Although the resulting rotation matrix is a rotation matrix that isorthogonal for any N, the full modulation diversity is only achievedwhen N is a power of two.

Each of these methods can guarantee a certain degree of diversity.However, the resulting rotation matrix is fixed, having no parameterthat allows the optimization for different constellation sizes.Therefore, a severe disadvantage of these methods is that the effect ofmodulation diversity cannot be maximized in accordance with differentconstellation sizes.

The second approach first constructs rotation matrices with two andthree dimensions, which can be used as base matrices for constructingmatrices with more dimensions using a Hadamard-like stacked expansionshown in the following Math. 11.

$\begin{matrix}{R = \begin{pmatrix}{+ R_{1}} & {- R_{2}} \\{+ R_{2}} & {+ R_{1}}\end{pmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 11} \right\rbrack\end{matrix}$

The base 2-D and 3-D (three-dimensional) rotation matrices have a singleindependent parameter which is chosen so that the product distance ofthe constellation is maximized. A 4-D rotation matrix is constructedfrom two 2-D rotation matrices according to the above Math. 11. Becauseof the relative small dimension, it is possible to find an algebraicrelationship between parameters of the two 2-D rotation matrices, sothat the product distance is maximized. For larger dimensions, such anoptimization becomes intractable, which is the primary disadvantage ofthe second approach.

(Mapping Constellation Components to Ensure Independent Fading)

Another aspect concerns the separation and mapping of the N dimensionsof the rotated constellation so that they experience independent fading.This is a key aspect necessary for achieving the expected diversityperformance.

The N constellation components, which are obtained by separating theN-dimensional rotated constellation on a per-dimension basis, can betransmitted over different time slots, frequencies, transmitterantennas, or combinations thereof. Further signal processing is possiblebefore transmission. The critical aspect is that fading experienced byeach of the N dimensions must be different from, or ideally uncorrelatedwith, fading experienced by any other one of the N dimensions.

The spreading of the N dimensions across different time slots,frequencies and antennas can be achieved for example through appropriateinterleaving and mapping.

(Mapping Constellation Components to Transmitted Complex Cells)

Another aspect concerns the mapping of the N real dimensions of therotated constellation to complex symbols for transmission. In order toensure the desired diversity, the N dimensions must be mapped todifferent complex symbols. The complex symbols are then spread asdescribed earlier, e.g. through interleaving and mapping, so that at thereception, fading experienced by each of the N dimensions isuncorrelated with fading of any other one of the N dimensions.

FIG. 2 is a block diagram of a transmission apparatus.

The transmission apparatus is composed of an FEC encoder 210, a bitinterleaver 220, a rotated constellation mapper 230, a complex symbolmapper 240, a symbol interleaver/mapper 250, modulation chains 260-1 to260-M, and transmitter antennas 270-1 to 270-M.

The FEC encoder 210 performs forward error correction (FEC) encoding onthe input thereto. Note that the best FEC codes known so far, which arealso the most used in new standards, are the turbo codes and thelow-density parity check (LDPC) codes.

The bit interleaver 220 performs bit interleaving on the input from theFEC encoder 210. Here, the bit interleaving can be block interleaving orconvolution interleaving.

The rotated constellation mapper 230 maps the input from the bitinterleaver 220 to the rotated constellation.

Generally, the input to the rotated constellation mapper 230 is theoutput of the FEC encoder 210 via the bit interleaver 220 that performsoptional bit interleaving. The bit interleaving is usually required whenthere are more than one bit per dimension (B>1). The FEC encodingperformed by the FEC encoder 210 introduces redundant bits in acontrolled fashion, so that propagation errors can be corrected in thereception apparatus. Although the overall spectral efficiency decreases,the transmission becomes overall more robust, i.e., the bit error rate(BER) decays much faster with the signal to noise ratio (SNR).

Note that regarding the original mapping of the information bits on thenon-rotated hyper-cubic constellations, each dimension is modulatedseparately by B bits, using either binary or Gray mapping, so the numberof discrete values is 2^(B) and the number of constellation points is2^(B*N).

The complex symbol mapper 240 maps each of N constellation components,which represent N-dimensional rotated constellation symbols input fromthe rotated constellation mapper 230, to a different one of complexsymbols.

There are multiple possibilities for the mapping performed by thecomplex symbol mapper 240, i.e., the mapping of each of N constellationcomponents, which represent N-dimensional rotated constellation symbols,to a different one of complex symbols. Some of such possibilities areillustrated in FIG. 3. The essential function of the complex symbolmapper 240 is to map each of N constellation components of one rotatedconstellation symbol to a different one of complex symbols.

By way of example, FIG. 3 shows the case of four dimensions. Referringto FIG. 3, the boxes showing the same number (e.g., “1”) represent agroup of 4-D rotated constellation symbols. The number shown by each boxindicates the group number of the corresponding group. Also, each boxindicates a constellation component of one dimension.

Shown below “Constellation symbols” in FIG. 3 is a state where sixgroups of 4-D rotated constellation symbols are aligned. Shown below“Complex symbols” in FIG. 3 are twelve complex symbols, which areobtained by rearranging the six groups of 4-D rotated constellationsymbols shown below “Constellation symbols” in FIG. 3. Note that FIG. 3shows three forms of “Complex symbols” as examples. At the time ofactual transmission, a pair of two constellation components that arevertically aligned below “Complex symbols” (the result of rearrangement)is modulated and transmitted as one complex symbol.

The symbol interleaver/mapper 250 performs symbol interleaving on thecomplex symbols input from the complex symbol mapper 240, and thereaftermaps the complex symbols to different time slots, frequencies,transmitter antennas, or combinations thereof. Here, the symbolinterleaving can be block interleaving or convolution interleaving.

The modulation chains 260-1 to 260-M are provided in one-to-onecorrespondence with the transmitter antennas 270-1 to 270-M. Each of themodulation chains 260-1 to 260-M inserts pilots for estimating thefading coefficients into the corresponding input from the symbolinterleaver/mapper 250, and also performs various processing, such asconversion into the time domain, digital-to-analog (D/A) conversion,transmission filtering and orthogonal modulation, on the correspondinginput. Then, each of the modulation chains 260-1 to 260-M transmits thetransmission signal via a corresponding one of the transmitter antennas270-1 to 270-M.

(Receiver Side)

On the receiver side, the exact inverse steps of the steps performed bythe transmission apparatus must be performed. FIG. 4 shows a blockdiagram of a reception apparatus corresponding to the transmissionapparatus whose block diagram is shown in FIG. 2.

The reception apparatus is composed of receiver antennas 410-1 to 410-M,demodulation chains 420-1 to 420-M, a symbol demapper/deinterleaver 430,a complex symbol demapper 440, a rotated constellation demapper 450, abit deinterleaver 460, and an FEC decoder 470.

The demodulation chains 420-1 to 420-M are provided in one-to-onecorrespondence with the receiver antennas 410-1 to 410-M. Each of thedemodulation chains 420-1 to 420-M performs processing such as A/Dconversion, reception filtering, and orthogonal demodulation on thesignal transmitted by the transmission apparatus of FIG. 2 and receivedby a corresponding one of the receiver antennas 410-1 to 410-M. Then,the demodulation chains 420-1 to 420-M estimate (i) the amplitude values(fading coefficients) of the channel characteristics by using the pilotsand (ii) noise variance, and output the estimated amplitude values andnoise variance together with the phase-corrected received signal.

The symbol demapper/deinterleaver 430 performs the inverse processing ofthe processing performed by the symbol interleaver/mapper 230 in thetransmission apparatus on the inputs from the demodulation chains 420-1to 420-M.

The complex symbol demapper 440 performs the inverse processing of theprocessing performed by the complex symbol mapper 240 in thetransmission apparatus on the input from the symboldemapper/deinterleaver 430. Through this processing, N-dimensionalrotated constellation symbols can be obtained.

The rotated constellation demapper 450 performs demapping processing onthe N-dimensional rotated constellation symbols, and outputs a decisionresult of each bit included in the N-dimensional rotated constellation.

The bit deinterleaver 460 performs the inverse processing of theprocessing performed by the bit interleaver 220 in the transmissionapparatus on the input from the rotated constellation demapper 450.

The FEC decoder 470 performs FEC decoding on the input from the bitdeinterleaver 470.

Below, further explanations of the rotated constellation demapper 450are given.

The rotated constellation demapper 450 can perform the processing ofdemapping N-dimensional rotated constellation symbols in the followingtwo ways (i) and (ii).

(i) First de-rotate the constellation, then extract the bits for eachdimension separately.

(ii) Decode the bits of all dimensions in one step.

Although the first solution (the above (i)) is the most simple, itsperformance is suboptimal and even worse for rotated constellations thanfor non-rotated constellations. Due to its simplicity, this solution maybe used in some low-cost reception apparatuses.

Although the second solution (the above (ii)) is more complex, it offersmuch better performance in terms of BER at a given SNR. In thefollowing, the second solution will be described in greater detail.

As with the transmission apparatus, a preferred embodiment of thereception apparatus includes the FEC decoder 470 after the rotatedconstellation demapper 450, with the optional bit deinterleaver 460 inbetween, as shown in FIG. 4. More exactly, the rotated constellationdemapper 450, which performs the rotated constellation demapping,receives N-dimensional symbol vectors (y₁, . . . , y_(N)) and theestimated fading coefficient vectors (h₁, . . . , h_(N)), and extractsdata of N*B bits (b₁, . . . , b_(N*B)) from each symbol, as shown inFIG. 5.

When FEC decoding is used, the processing of demapping the N-dimensionalrotated constellation symbols can no longer be performed by way of ahard decision, because the performance of the error correction would besuboptimal. Instead, “soft bits” must be used, either in the form ofprobabilities or in the form of log-likelihood ratios (LLRs). The LLRrepresentation is preferred because probability multiplications can beconveniently expressed as sums. By definition, the LLR of a bit b_(k) isshown in the following Math. 12.

$\begin{matrix}{{L\left( {b_{k}❘y} \right)}\overset{\bigtriangleup}{=}{\ln\frac{P\left( {b_{k} = {1❘y}} \right)}{P\left( {b_{k} = {0❘y}} \right)}}} & \left\lbrack {{Math}.\mspace{14mu} 12} \right\rbrack\end{matrix}$

Note that in Math. 12,

-   -   P(b_(k)=0|y)        and    -   P(b_(k)=1|y)        are the a-priori probabilities that b_(k)=0 and b_(k)=1 were        transmitted when the symbol vector

-       is received. According to the known theory, the LLR of a bit b_(k)    of a constellation has the exact expression shown in the following    Math. 13.

$\begin{matrix}{{L\left( b_{k} \right)} = {\ln\frac{\sum\limits_{s \in S_{k}^{1}}^{\;}\;{\exp\left( {- \frac{{{y - {Hs}}}^{2}}{2\;\sigma^{2}}} \right)}}{\sum\limits_{s \in S_{k}^{0}}^{\;}\;{\exp\left( {- \frac{{{y - {Hs}}}^{2}}{2\;\sigma^{2}}} \right)}}}} & \left\lbrack {{Math}.\mspace{14mu} 13} \right\rbrack\end{matrix}$

Note that in Math. 13, k is the bit index,

-       is the received symbol vector,-       is the diagonal matrix having the associated (estimated) fading    coefficients as elements on the main diagonal,-       is a constellation point vector,    -   ∥∥²        is the squared norm, and    -   σ²        is the noise variance.

For an N-dimensional constellation, the squared norm represents thesquared Euclidean distance from the received symbol vector

-       to the faded constellation symbol vector-       in the N-dimensional space. The squared norm can be expressed by the    following Math. 14.

$\begin{matrix}{{{y - {Hs}}}^{2} = {\sum\limits_{n = 1}^{N}\;{{y_{n} - {h_{n}s_{n}}}}^{2}}} & \left\lbrack {{Math}.\mspace{14mu} 14} \right\rbrack\end{matrix}$

Each bit b_(k) divides the constellation into two partitions of equalsize, S_(k) ⁰ and S_(k) ¹, corresponding to those points for which b_(k)is 0 and 1, respectively. Examples are shown in FIGS. 6A and 6B for aclassical 16-QAM constellation with Gray encoding. FIG. 6A shows theconstellation encoding and FIG. 6B shows the two partitions for each bitb_(k).

The exact expression for the LLR (the above Math. 13) is difficult tocalculate due to the exponentials, divisions and the logarithm. Inpractice, the approximation shown in the following Math. 15 is made,called max-log, which introduces negligible errors.ln(e ^(a) ¹ +e ^(a) ² )≈max(a ₁ ,a ₂)→ln(e ^(−a) ¹ +e ^(−a) ² )≈min(a ₁,a ₂)  [Math. 15]

By using the above Math. 15, the above Math. 13 leads to a much moresimple expression for the LLR, which is shown in the following Math. 16.

$\begin{matrix}{{L\left( b_{k} \right)} \approx {{\frac{1}{2\;\sigma^{2}}{\min\limits_{s \in S_{k}^{0}}\mspace{14mu}{{y - {Hs}}}^{2}}} - {\frac{1}{2\;\sigma^{2}}{\min\limits_{s \in S_{k}^{1}}\mspace{14mu}{{y - {Hs}}}^{2}}}}} & \left\lbrack {{Math}.\mspace{14mu} 16} \right\rbrack\end{matrix}$

For each received symbol vector

-   ,    the distances to all 2^(B*N) constellation points must be    calculated, and the corresponding minimum for each partition is    determined.

FIG. 7 shows a preferred hardware implementation of an LLR demapper (oneexample of the rotated constellation demapper 450 shown in FIG. 4) for a16-QAM rotated constellation (N=2, B=2).

The LLR demapper is composed of a counter 710, a rotated constellationmapper 720, a squared Euclidean distance calculator 730, minimizers740-1 to 740-4, and adders 750-1 to 750-4.

For each received symbol vector

-   ,    the counter 710 repeatedly generates all 2⁴=16 constellation points,    and outputs four bits b₁, b₂, b₃ and b₄ indicating the constellation    points to the rotated constellation mapper 720.

The rotated constellation mapper 720 selects the 2-D rotatedconstellation point from a look-up table by using the counter valuesprovided by the counter 710 as an indexes, and outputs two constellationcomponents s₁ and s₂ obtained through this selection to the squaredEuclidean distance calculator 730.

The squared Euclidean distance calculator 730 calculates the squaredEuclidean distances (see FIG. 8).

For each bit, the minimizers 740-1 to 410-4 maintain the correspondingminimum squared Euclidean distances for the two partitions (see FIG. 9).The two constellation partitions for each bit are simply indicated bythe corresponding bit of the counter 710.

Each of the adders 750-1 to 750-4 subtracts the output of min1(corresponding to bit 1) from the output of min0 (corresponding to bit0), the min1 and min0 being provided in each of the minimizers 740-1 to740-4. Thereafter, the adders 750-1 to 750-4 output the results of thesubtraction as L(b₁) to L(b₄), respectively.

FIG. 8 is a circuit diagram of a squared Euclidean distance calculatorthat calculates an N-dimensional squared Euclidean distance. Note thatthe circuit structure of the squared Euclidean distance calculator 730has been modified from the one shown in FIG. 8 so as to satisfy N=2.

The squared Euclidean distance calculator is composed of multipliers810-1 to 810-N, adders 820-1 to 820-N, multipliers 830-1 to 830-N, anadder 840, and a multiplier 850.

The multipliers 810-1 to 810-N multiply h₁ to h_(N) by s₁ to s_(N),respectively. The adders 820-1 to 820-N subtract h₁s₁ to h_(N)s_(N) fromy₁ to y_(N), respectively. The multipliers 830-1 to 830-N multiply(y₁−h₁s₁) to (y_(N)−h_(N)s_(N)) by (y₁−h₁s₁) to (y_(N)−h_(N)s_(N)),respectively.

The adder 840 adds together the outputs of the multipliers 830-1 to830-N. The multiplier 850 multiplies the output of the adder 840 by

-   1/(2σ²).    The output of the multiplier 850 is the N-dimensional squared    Euclidean distance.

FIG. 9 is a circuit diagram of the minimizers 740-1 to 740-4 that eachcalculate the minimum squared Euclidean distances for each bit. The1-bit subset (or partition) input indicates the current position.

Each of the minimizers 740-1 to 740-4 is composed of a comparator 910, aselector 920, an inverter 930, D flip-flops 940-0 and 940-1, and aselector 950.

The following describes the operations to be performed in the situationof FIG. 9 when the subset value (the value input from the counter 710)is “0”.

From among the output of the D flip-flop 940-0 and the output of the Dflip-flop 940-1, the selector 950 selects and outputs the former.

The comparator 910 compares din (A), which indicates the squaredEuclidean distance calculated by the squared Euclidian distancecalculator 730, with the output (B) of the selector 950. In a case whereB is smaller than A, the comparator 910 outputs “0”. In this case, fromamong din and the output of the selector 950, the selector 920 selectsand outputs the latter based on “0” received from the comparator 910. Onthe other hand, in a case where A is smaller than B, the comparator 910outputs “1”. In this case, from among din and the output of the selector950, the selector 920 selects and outputs the former based on “1”received from the comparator 910. Note that in a case where A is equalto B, the same result will be obtained whether the selector 920 selectsdin or the output of the selector 950. Accordingly, in this case, thecomparator 910 may output either one of “0” and “1”.

The inverter 930 inverts the subset value “0”. Thus, “1” is input to theenable terminal of the D flip-flop 940-0. As the D flip-flop 940-0 isenabled, it latches the output of the selector 920. Meanwhile, “0” isinput to the enable terminal of the D flip-flop 940-1. As the Dflip-flop 940-1 is disabled, it does not latch the output of theselector 920.

The following describes the operations to be performed in the situationof FIG. 9 when the subset value is “1”.

From among the output of the D flip-flop 940-0 and the output of the Dflip-flop 940-1, the selector 950 selects and outputs the latter.

The comparator 910 compares din (A) with the output (B) from theselector 950. In a case where B is smaller than A, the comparator 910outputs “0”. In this case, from among din and the output of the selector950, the selector 920 selects and outputs the latter based on “0”received from the comparator 910. On the other hand, in a case where Ais smaller than B, the comparator 910 outputs “1”. In this case, fromamong din and the output of the selector 950, the selector 920 selectsand outputs the former based on “1” received from the comparator 910.Note that in a case where A is equal to B, the same result will beobtained whether the selector 920 selects din or the output of theselector 950. Accordingly, in this case, the comparator 910 may outputeither one of “0” and “1”.

“1” is input to the enable terminal of the D flip-flop 940-1. As the Dflip-flop 940-1 is enabled, it latches the output of the selector 920.Meanwhile, the inverter 930 inverts the subset value “1”. Thus, “0” isinput to the enable terminal of the D flip-flop 940-0. As the Dflip-flop 940-0 is disabled, it does not latch the output of theselector 920.

A significant improvement in performance of the reception apparatus canbe achieved by using iterative decoding. As shown in FIG. 10, thereception apparatus configured to utilize such iterative decoding iscomposed of a rotated constellation demapper 1010, a bit deinterleaver1020, an FEC decoder 1030, an adder 1040, and a bit interleaver 1050.Here, the rotated constellation demapper 1010 and the FEC decoder 1030are connected in a loop.

The rotated constellation demapper 1010 performs demapping processing onN-dimensional rotated constellation symbols, and outputs L (see FIG.11). The bit deinterleaver 1020 performs the inverse processing of theprocessing performed by the bit interleaver 220 in the transmissionapparatus on the input from the rotated constellation demapper 1010. TheFEC decoder 1030 performs FEC decoding on the input from the bitdeinterleaver 1020.

The adder 1040 subtracts the input from the FEC decoder 1030 from theoutput of the FEC decoder 1030. The bit interleaver 1050 performs thesame processing as the processing performed by the bit interleaver 220in the transmission apparatus on the output of the adder 1040, and thenoutputs L_(E). L_(E), also referred to as extrinsic information, is fedback to the rotated constellation demapper 1010 in order to aid thedemapping processing performed by the rotated constellation demapper1010, i.e., the processing of demapping the N-dimensional rotatedconstellation symbols. In this case it is essential that the FECdecoding produces soft bits, e.g. in the form of LLRs.

As known in the literature, the formula for calculating the LLR for bitb_(k) is given by the following Math. 17.

$\begin{matrix}{{L\left( b_{k} \right)} \approx {{\min\limits_{x \in X_{k}^{0}}\left\{ {{\frac{1}{2\sigma^{2}}{{y - {{Hs}(x)}}}^{2}} + {\sum\limits_{\underset{{x_{i} = 1},{i \neq j}}{i = 1}}^{K}{L_{E}\left( b_{i} \right)}}} \right\}} - {\min\limits_{x \in X_{k}^{1}}\left\{ {{\frac{1}{2\sigma^{2}}{{y - {{Hs}(x)}}}^{2}} + {\sum\limits_{\underset{{x_{i} = 1},{i \neq j}}{i = 1}}^{K}{L_{E}\left( b_{i} \right)}}} \right\}}}} & \left\lbrack {{Math}.\mspace{14mu} 17} \right\rbrack\end{matrix}$

In Math. 17,

-   -           represents the K=N*B bits associated with each constellation        point, and X_(k) ⁰ and X_(k) ¹ represent the two constellation        partitions associated with bit k, each constellation point being        represented by the N*B bits instead of the N bits of integer        coordinates. Furthermore,    -           is expressed as    -           and represents the constellation mapping function.

For example, X₃ ⁰ and X₃ ¹ are shown in the following Math. 18.

$\begin{matrix}\begin{matrix}X_{3}^{0} & X_{3}^{1} \\0000 & 0100 \\0001 & 0101 \\0010 & 0110 \\0011 & 0111 \\1000 & 1100 \\1001 & 1101 \\1010 & 1110 \\1011 & 1111\end{matrix} & \left\lbrack {{Math}.\mspace{14mu} 18} \right\rbrack\end{matrix}$

FIG. 11 shows an example of the structure of the rotated constellationdemapper 1010 for iterative decoding. Note that the rotatedconstellation demapper 1010 for iterative decoding is similar to arotated constellation demapper for non-iterative decoding. Below, theelements that are the same as those described above are assigned thesame reference numerals thereas, and a detailed description thereof isomitted.

The rotated constellation demapper 1010 is composed of a counter 710, arotated constellation mapper 720, a squared Euclidean distancecalculator 730, minimizers 740-1 to 740-4, adders 750-1 to 750-4,logical AND operators 1110-1 to 1110-4, an adder 1120, adders 1130-1 to1130-4, and adders 1140-1 to 1140-4.

The logical AND operators 1110-1 to 1110-4 perform logical ANDoperations on the outputs of the bit interleaver 1050, namely L_(E)(b1)to L_(E)(b4), and the outputs of the counter 710, namely b1 to b4. Theadder 1120 adds together the outputs of the logical AND operators 1110-1to 1110-4. Each of the adders 1130-1 to 1130-4 subtracts, from theoutput of the adder 1120, the output of a corresponding one of thelogical AND operators 1110-1 to 1110-4. Each of the adders 1140-1 to1140-4 subtracts, from the output of the squared Euclidean distancecalculator 730, the output of a corresponding one of the adders 1130-1to 1130-4. Then, each of the adders 1140-1 to 1140-4 outputs the valueobtained through the subtraction to din of a corresponding one of theminimizers 740-1 to 740-4.

CITATION LIST Non Patent Literature

-   [NPL 1]

K. Boulle and J. C. Belfiore. “Modulation Scheme Designed for theRayleigh Fading Channel.” Presented at CISS 1992.

-   [NPL 2]

B. D. Jelicic and S. Roy. “Design of Trellis Coded QAM for Flat Fadingand AWGN Channels.” IEEE Transactions on Vehicular Technology, Vol. 44.February 1995.

-   [NPL 3]

J. Boutros and E. Viterbo. “Signal Space Diversity: A Power- andBandwidth-Efficient Diversity Technique for the Rayleigh FadingChannel.” IEEE Transactions on Information Theory, Vol. 44. July 1998.

-   [NPL 4]    -   M. O. Damen, K. Abed-Meraim, and J. C. Belfiore. “Diagonal        Algebraic Space-Time Block Codes.” IEEE Transactions on        Information Theory, Vol. 48. March 2002.

SUMMARY OF INVENTION Technical Problem

As described above, although a wide range of proposals have been maderegarding rotation matrices for rotating a constellation, the proposalsthat have been made so far do not provide any efficient method ofgenerating a multi-dimensional rotated constellation (amulti-dimensional rotation matrix) for digital modulation with a highdegree of modulation diversity with respect to various constellationsizes.

NPL 2 introduces an approach that makes use of a Givens rotation. Theproblem with this approach is that the number of parameters forgenerating an optimal multi-dimensional rotated constellation increasesby the order of the square of the number of dimensions in theconstellation.

NPL 3 introduces two approaches. The first approach makes use ofcanonical embedding. According to this approach, the method ofgenerating a multi-dimensional rotation matrix is uniquely determinedbased on the number of dimensions, and does not have a parameterenabling the optimization for different constellation sizes. Therefore,the problem with this approach is that it does not allow maximizing theeffect of modulation diversity for various constellation sizes.

The second approach introduced by NPL 3 generates a multi-dimensionalrotation matrix having a larger number of dimensions by using stackedexpansion where 2-D and 3-D rotation matrices are stacked. The problemwith this approach is that the algebraic relationships between thestacked rotation matrices become more complicated as the number ofdimensions increases, rendering the optimization difficult.

It is the object of the present invention to provide an efficient methodof generating a multi-dimensional rotated constellation (amulti-dimensional rotation matrix) for digital transmission with a highdegree of modulation diversity with respect to various constellationsizes. It is also the object of the present invention to provide atransmission apparatus and a transmission method for transmitting databased on the multi-dimensional rotated constellation obtained by usingthe above method, and a reception apparatus and a reception method forreceiving data based on the multi-dimensional rotated constellationobtained by using the above method.

Solution to Problem

A transmission apparatus of the present invention transmits a block ofdata over a plurality of transmission channels. The transmissionapparatus comprises: a modulator operable to select one of a pluralityof constellation points in accordance with the block of data to betransmitted, each of the plurality of constellation points having aplurality of components; and a transmitter operable to transmit eachcomponent of the selected constellation point over a different one ofthe plurality of transmission channels, wherein (i) the plurality ofconstellation points are defined by positions thereof within anN-dimensional space, the positions being obtained by applying anorthogonal transformation to a subset of

-   -   ^(N ,)        which is an N-dimensional integer lattice, (ii) N is a multiple        of four, and (iii) the orthogonal transformation has an N-by-N        matrix representation with absolute values of all elements on a        main diagonal equal to a first value, and with absolute values        of all elements not on the main diagonal equal to a non-zero        second value.

Advantageous Effects of Invention

The above transmission apparatus allows efficiently generating amulti-dimensional rotated constellation (a multi-dimensional rotationmatrix) for digital transmission with a high degree of modulationdiversity with respect to various constellation sizes. Due to themulti-dimensional rotated constellation obtained by using the generatedmulti-dimensional rotation matrix, the above transmission apparatus alsoenables data transmission that yields the effect of a high-degree ofmodulation diversity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A illustrates an example constellation in 2D and the effect offading.

FIG. 1B illustrates an example constellation in 2D that is obtained byrotating the constellation of FIG. 1A and the effect of fading.

FIG. 2 shows a block diagram of a conventional transmission apparatus.

FIG. 3 is a schematic drawing illustrating the mapping of constellationsymbols to complex symbols.

FIG. 4 is a block diagram of a conventional reception apparatus.

FIG. 5 is an illustration of the inputs to and the outputs of a rotatedconstellation demapper.

FIG. 6A shows an example of a conventional 16-QAM constellation withGray encoding.

FIG. 6B illustrates the two partitions for each bit of the constellationof FIG. 6A.

FIG. 7 shows an example hardware implementation of an LLR demapper for a16-QAM rotated constellation.

FIG. 8 shows an example hardware implementation for a squared Euclideandistance calculator that calculates the N-dimensional squared Euclideandistance.

FIG. 9 shows an example hardware implementation for a minimizer thatcalculates the minimum squared Euclidean distances.

FIG. 10 shows a block diagram of a circuit that performs iterativedecoding.

FIG. 11 shows an example hardware implementation of the rotatedconstellation demapper for iterative decoding.

FIG. 12 shows a block diagram of a transmission apparatus according toan embodiment of the present invention.

FIG. 13 shows a block diagram of a reception apparatus according to anembodiment of the present invention.

FIG. 14 is a block diagram of the rotated constellation demapper shownin FIG. 13.

DESCRIPTION OF EMBODIMENTS

The present invention provides a first transmission apparatus fortransmitting a block of data over a plurality of transmission channels,the first transmission apparatus comprising: a modulator operable toselect one of a plurality of constellation points in accordance with theblock of data to be transmitted, each of the plurality of constellationpoints having a plurality of components; and a transmitter operable totransmit each component of the selected constellation point over adifferent one of the plurality of transmission channels, wherein (i) theplurality of constellation points are defined by positions thereofwithin an N-dimensional space, the positions being obtained by applyingan orthogonal transformation to a subset of

-   -   ^(N ,)        which is an N-dimensional integer lattice, (ii) N is a multiple        of four, and (iii) the orthogonal transformation has an N-by-N        matrix representation with absolute values of all elements on a        main diagonal equal to a first value, and with absolute values        of all elements not on the main diagonal equal to a non-zero        second value.

The present invention also provides a first transmission method fortransmitting a block of data over a plurality of transmission channels,the first transmission method comprising the steps of: selecting one ofa plurality of constellation points in accordance with the block of datato be transmitted, each of the plurality of constellation points havinga plurality of components; and transmitting each component of theselected constellation point over a different one of the plurality oftransmission channels, wherein (i) the plurality of constellation pointsare defined by positions thereof within an N-dimensional space, thepositions being obtained by applying an orthogonal transformation to asubset of

-   -   ^(N ,)        which is an N-dimensional integer lattice, (ii) N is a multiple        of four, and (iii) the orthogonal transformation has an N-by-N        matrix representation with absolute values of all elements on a        main diagonal equal to a first value, and with absolute values        of all elements not on the main diagonal equal to a non-zero        second value.

The above transmission apparatus and transmission method allowefficiently generating a multi-dimensional rotated constellation (amulti-dimensional rotation matrix) for digital transmission with a highdegree of modulation diversity with respect to various constellationsizes. Due to the multi-dimensional rotated constellation obtained byusing the generated multi-dimensional rotation matrix, the abovetransmission apparatus and transmission method also enable datatransmission that yields the effect of a high-degree of modulationdiversity.

The present invention also provides a second transmission apparatus anda second transmission method, which are the first transmission apparatusand the second transmission apparatus, respectively, wherein instead ofthe N-by-N matrix representation, the orthogonal transformation has amatrix representation obtained by permuting rows and/or columns in theN-by-N matrix representation.

The above structure produces the same effect as the effect produced bythe N-by-N matrix representation with absolute values of all elements onthe main diagonal equal to a first value, and with absolute values ofall elements not on the main diagonal equal to a non-zero second value.

The present invention also provides a third transmission apparatus,which is the first transmission apparatus further comprising a mapperoperable to map each component of the selected constellation point tothe corresponding one of the plurality of transmission channels overwhich the component is to be transmitted, such that fading of each ofthe plurality of transmission channels is uncorrelated with fading ofany other one of the plurality of transmission channels.

The present invention also provides a third transmission method, whichis the first transmission method further comprising the step of mappingeach component of the selected constellation point to the correspondingone of the plurality of transmission channels over which the componentis to be transmitted, such that fading of each of the plurality oftransmission channels is uncorrelated with fading of any other one ofthe plurality of transmission channels.

The above structure can optimize the transmission performance, even inthe presence of fading.

The present invention also provides a fourth transmission apparatus,which is the first transmission apparatus wherein the transmitter isadapted for transmitting each component of the selected constellationpoint over a different one of a plurality of time slots, frequencies,transmitter antennas, or combinations thereof.

The present invention also provides a fifth transmission apparatus and afourth transmission method, which are the first transmission apparatusand the first transmission method, respectively, wherein the pluralityof transmission channels comprise a plurality of different carriers inan orthogonal frequency-division multiplexing scheme.

The present invention also provides a sixth transmission apparatus and afifth transmission method, which are the first transmission apparatusand the first transmission method, respectively, wherein the pluralityof transmission channels comprise a plurality of different symbols in anorthogonal frequency-division multiplexing scheme.

The present invention also provides a first reception apparatus forreceiving a block of data over a plurality of transmission channels, thefirst reception apparatus comprising: a receiver operable to receive aplurality of component signals over the plurality of transmissionchannels; and a demodulator operable to select one of a plurality ofconstellation points in accordance with the plurality of receivedcomponent signals, wherein (i) the plurality of constellation points aredefined by positions thereof within an N-dimensional space, thepositions being obtained by applying an orthogonal transformation to asubset of

-   ^(N ,)    which is an N-dimensional integer lattice, (ii) N is a multiple of    four, and (iii) the orthogonal transformation has an N-by-N matrix    representation with absolute values of all elements on a main    diagonal equal to a first value, and with absolute values of all    elements not on the main diagonal equal to a non-zero second value.

The present invention also provides a first reception method forreceiving a block of data over a plurality of transmission channels, thefirst reception method comprising the steps of: receiving a plurality ofcomponent signals over the plurality of transmission channels; andselecting one of a plurality of constellation points in accordance withthe plurality of received component signals, wherein (i) the pluralityof constellation points are defined by positions thereof within anN-dimensional space, the positions being obtained by applying anorthogonal transformation to a subset of

-   ^(N ,)    which is an N-dimensional integer lattice, (ii) N is a multiple of    four, and (iii) the orthogonal transformation has an N-by-N matrix    representation with absolute values of all elements on a main    diagonal equal to a first value, and with absolute values of all    elements not on the main diagonal equal to a non-zero second value.

The above reception apparatus and reception method allow efficientlygenerating a multi-dimensional rotated constellation (amulti-dimensional rotation matrix) for digital transmission with a highdegree of modulation diversity with respect to various constellationsizes. Due to the multi-dimensional rotated constellation obtained byusing the generated multi-dimensional rotation matrix, the abovereception apparatus and reception method also enable data reception thatyields the effect of a high-degree of modulation diversity.

The present invention also provides a second reception apparatus and asecond reception method, which are the first reception apparatus and thefirst reception method, respectively, wherein instead of the N-by-Nmatrix representation, the orthogonal transformation has a matrixrepresentation obtained by permuting rows and/or columns in the N-by-Nmatrix representation.

The above structure produces the same effect as the effect produced bythe N-by-N matrix representation with absolute values of all elements onthe main diagonal equal to a first value, and with absolute values ofall elements not on the main diagonal equal to a non-zero second value.

The present invention also provides a third reception apparatus and athird reception method, which are the first reception apparatus and thefirst reception method, respectively, wherein the plurality oftransmission channels comprise a plurality of different carriers in anorthogonal frequency-division multiplexing scheme.

The present invention also provides a fourth reception apparatus and afourth reception method, which are the first reception apparatus and thefirst reception method, respectively, wherein the plurality oftransmission channels comprise a plurality of different symbols in anorthogonal frequency-division multiplexing scheme.

The present invention also provides a first generation method forgenerating a multi-dimensional constellation for a digital modulationscheme in a data communication system, the first generation methodcomprising the steps of: receiving a plurality of vectors of amulti-dimensional vector space; and obtaining constellation points ofthe multi-dimensional constellation by applying an orthogonaltransformation to the plurality of vectors received, wherein (i) theorthogonal transformation is adapted for increasing a minimum number ofdifferent values in components of any two distinct multi-dimensionalconstellation points relative to a minimum number of different values incomponents of any two distinct vectors received, and (ii) the orthogonaltransformation has an N-by-N matrix representation, N being a multipleof four, with absolute values of all elements on a main diagonal equalto a first value, and with absolute values of all elements not on themain diagonal equal to a non-zero second value.

The above generation method allows efficiently generating amulti-dimensional rotated constellation (a multi-dimensional rotationmatrix) for digital transmission with a high degree of modulationdiversity with respect to various constellation sizes.

The present invention also provides a second generation method forgenerating a multi-dimensional constellation, the second generationmethod being the first generation method wherein instead of the N-by-Nmatrix representation, the orthogonal transformation has a matrixrepresentation obtained by permuting rows and/or columns in the N-by-Nmatrix representation.

The above structure produces the same effect as the effect produced bythe N-by-N matrix representation with absolute values of all elements onthe main diagonal equal to a first value, and with absolute values ofall elements not on the main diagonal equal to a non-zero second value.

The present invention also provides a third generation method forgenerating a multi-dimensional constellation, the third generationmethod being the first generation method further comprising the stepsof: selecting a rotation factor r as a real number between 0 and 1;calculating the first value, a, by evaluating an expression

${a = \sqrt{1 - \frac{r\left( {N - 1} \right)}{N}}};$calculating the second value, b, by evaluating an expression

${b = \sqrt{\frac{r}{N}}};$and determining the orthogonal transformation by selecting a sign values_(i,j) for each element (i, j) of a matrix representation

${R = \begin{pmatrix}{s_{1,1}a} & {s_{1,2}b} & \ldots & {s_{1,N}b} \\{s_{2,1}b} & {s_{2,2}a} & \; & \vdots \\\vdots & \; & \ddots & {s_{{N - 1},N}b} \\{s_{N,1}b} & \ldots & {s_{N,{N - 1}}b} & {s_{N,N}a}\end{pmatrix}},$such that the matrix representation is orthogonal.

With the above structure, the orthogonal transformation can be easilydetermined.

The present invention also provides a fourth generation method forgenerating a multi-dimensional constellation, the fourth generationmethod being the third generation method wherein the selected rotationfactor r maximizes the minimum number of different values in thecomponents of any two distinct multi-dimensional constellation points.

The above structure makes it possible to achieve a high-degree ofmodulation diversity and therewith increased robustness in the presenceof fading, while preserving spectral efficiency.

The present invention also provides a fifth generation method forgenerating a multi-dimensional constellation, the fifth generationmethod being the first generation method wherein the plurality ofvectors received represent a subset of

-   -   ^(N ,)        which is an N-dimensional integer lattice.

The above structure is useful in a straightforward numericalimplementation.

The following describes an embodiment of the present invention withreference to the drawings.

First, a description is now given of proposed multi-dimensional rotationmatrices.

Multi-dimensional rotation matrices have a single independent parameterand a structure that is as regular as possible. The parameter can beconfigured in order to minimize the error probability for variousconstellation sizes. Specifically, the following two conditions (i) and(ii) are imposed on the multi-dimensional rotation matrix employed forobtaining a multi-dimensional rotated constellation.

(i) Each output must have a dominant input.

(ii) The remaining inputs must have equal weights.

The above conditions (i) and (ii) are fulfilled if the multi-dimensionalrotation matrix is of the form shown in the following Math. 19 (forN=4), or more generally, of the form shown in the following Math. 20.Note that the multi-dimensional rotation matrix shown in Math. 20 is anN-by-N matrix.

$\begin{matrix}{R = \begin{pmatrix}{s_{1,1}a} & {s_{1,2}b} & {s_{1,3}b} & {s_{1,4}b} \\{s_{2,1}b} & {s_{2,2}a} & {s_{2,3}b} & {s_{2,4}b} \\{s_{3,1}b} & {s_{3,2}b} & {s_{3,3}a} & {s_{3,4}b} \\{s_{4,1}b} & {s_{4,2}b} & {s_{4,3}b} & {s_{4,4}a}\end{pmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 19} \right\rbrack \\{R = \begin{pmatrix}{s_{1,1}a} & {s_{1,2}b} & \ldots & {s_{1,N}b} \\{s_{2,1}b} & {s_{2,2}a} & \; & \vdots \\\vdots & \; & \ddots & {s_{{N - 1},N}b} \\{s_{N,1}b} & \ldots & {s_{N,{N - 1}}b} & {s_{N,N}a}\end{pmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 20} \right\rbrack\end{matrix}$

Here, a and b denote real parameters, with each sign value s_(i,j)satisfying

S_(i, j) ∈ {−1, +1}.Note that values of the parameters a and b that fulfill the aboveconditions (i) and (ii) satisfy a relational expression a>b>0.

Obviously, the same advantages can be achieved by permuting rows and/orcolumns of the multi-dimensional rotation matrix shown in the aboveMath. 20. Therefore, the matrix shown in Math. 20 can be used as themulti-dimensional rotation matrix. Alternatively, it is also possible touse a matrix obtained by permuting rows and/or columns of the matrixshown in Math. 20 as the multi-dimensional rotation matrix. The matrixshown in Math. 20 and the matrix obtained by permuting rows and/orcolumns of the matrix shown in Math. 20 have the following features: (i)each row contains an element having a real parameter a; (ii) each columncontains an element having a real parameter a; and (iii) the rest of theelements in each row/column have a real parameter b.

The following describes normalization of the multi-dimensional rotationmatrix shown in the above Math. 20. Note that similar normalization canbe performed on a matrix (a multi-dimensional rotation matrix) obtainedby permuting rows and/or columns of the matrix shown in Math. 20.

The normalization condition establishes the relationship shown in thefollowing Math. 21 between parameters a and b.a ²+(N−1)b ²=1  [Math. 21]

Therefore, the multi-dimensional rotation matrix has only oneindependent parameter. In the following Math. 22, we define a “rotationfactor” r between 0 and 1.No rotation: r=0→b=0,a=1Maximum rotation: r=0→b=a=√{square root over (1/N)}  [Math. 22]

Therefore, the parameters a and b can be expressed in terms of the“rotation factor” r as shown in the following Math. 23.

$\begin{matrix}{{a = \sqrt{1 - \frac{r\left( {N - 1} \right)}{N}}},{b = \sqrt{\frac{r}{N}}}} & \left\lbrack {{Math}.\mspace{14mu} 23} \right\rbrack\end{matrix}$

The advantage of using the “rotation factor” r is that the range isalways 0 to 1 regardless of the number of dimensions. The optimal valuefor the “rotation factor” r depends on the constellation size, that is,the number of dimensions N and the number of bits B per dimension forsquare/cubic constellations. Note that the value of r satisfying theabove conditions (i) and (ii) is greater than 0 and smaller than 1.

The multi-dimensional rotation matrix for rotating a multi-dimensionalconstellation may be normalized or unnormalized.

The only open issue is what values the sign matrix

-       should take. The sign matrix-       is defined by the following Math. 24.

$\begin{matrix}{S = \begin{pmatrix}s_{1,1} & \ldots & s_{1,N} \\\vdots & \ddots & \vdots \\s_{N,1} & \ldots & s_{N,N}\end{pmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 24} \right\rbrack\end{matrix}$

A necessary condition, which is not sufficient however, is that the signmatrix

-       must be orthogonal, up to a scaling factor. Such matrices are known    in the literature as the Hadamard matrices. Because a and b in the    multi-dimensional rotation matrix-       are different, the additional condition shown in the following Math.    25 must be imposed.    s _(i,i) s _(i,j) =−s _(j,i) s _(j,j) for all i≠j  [Math. 25]

This condition ensures that any a*b product cancels out with thecorresponding b*a product.

If all elements on the main diagonal have the same sign, and each pairof elements that are symmetrical with respect to the main diagonal haveopposite signs, this condition is fulfilled. Examples of suchparticularly preferred sign matrices for the 4-D and 8-D(eight-dimensional) cases are shown in the following Math. 26 and Math.27, respectively.

$\begin{matrix}{S = \begin{pmatrix}{+ 1} & {- 1} & {- 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {+ 1} \\{+ 1} & {+ 1} & {+ 1} & {- 1} \\{+ 1} & {- 1} & {+ 1} & {+ 1}\end{pmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 26} \right\rbrack \\{S = \begin{pmatrix}{+ 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} & {- 1} \\{+ 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {+ 1} & {- 1} & {+ 1} \\{+ 1} & {+ 1} & {+ 1} & {- 1} & {- 1} & {- 1} & {+ 1} & {+ 1} \\{+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} \\{+ 1} & {+ 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} & {- 1} & {- 1} \\{+ 1} & {- 1} & {+ 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} \\{+ 1} & {+ 1} & {- 1} & {- 1} & {+ 1} & {+ 1} & {+ 1} & {- 1} \\{+ 1} & {- 1} & {- 1} & {+ 1} & {+ 1} & {- 1} & {+ 1} & {+ 1}\end{pmatrix}} & \left\lbrack {{Math}.\mspace{14mu} 27} \right\rbrack\end{matrix}$

It is to be noted that Hadamard matrices are only possible for sizesthat are multiples of four. Therefore, multi-dimensional rotationmatrices exist only for numbers of dimensions that are multiples offour. Thus, the number of dimensions of a constellation according to thepresent invention is preferably a multiple of four (e.g., 4, 8, 12 and16).

Once the sign matrix

-       has been fixed, the resulting multi-dimensional rotation matrix-       may be optimized for a certain constellation size, i.e., the number    of bits or constellation points per dimension, by performing the    following steps: selecting the “rotation factor” r accordingly; and    calculating parameters a and b by substituting the selected    “rotation factor” r into the above Math. 23. To this end, any    suitable optimization algorithm may be employed. As an optimization    target, the minimum number of different values in the components of    any two distinct multi-dimensional rotated constellation points may    be employed. Other optimization targets may be used as well.    According to a preferred embodiment of the present invention, a cost    function is defined that takes the minimum absolute differences    between corresponding components of any two distinct    multi-dimensional rotated constellation points into account. An    example of such a cost function calculates the minimum over all N    absolute differences between corresponding components of two    multi-dimensional rotated constellation points and sums these    minimum values, or their squares over all pairs of multi-dimensional    rotated constellation points.

The multi-dimensional rotated constellation may already be useful if theminimum number of different values in the components of any two distinctmulti-dimensional rotated constellation points is larger than thatpertaining to the multi-dimensional unrotated constellation. Also, themulti-dimensional rotated constellation may already be useful if theminimum absolute difference of two corresponding components of any twodistinct multi-dimensional rotated constellation points is larger thanthat pertaining to the multi-dimensional unrotated constellation.

In a preferred embodiment of the present invention, the entiretransmission process including the transmission channel and the decoderis simulated in order to determine the bit error rate. The “rotationfactor” r may then be adapted so as to minimize the determined bit errorrate.

Hence, the present invention allows generating a multi-dimensionalrotated constellation that can be used for modulating and transmittingdata over a plurality of fading (sub-) channels or slots at optimumspectral efficiency. To this end, a conventional hyper-cubicconstellation with the desired number of dimensions N and the desirednumber of bits per dimension (i.e., the number of constellation pointsper direction) is set up, for instance, by selecting an appropriatesubset of

-   -   ^(N ,)        which is the N-dimensional integer lattice. Here,    -   ^(N)        is the set of all points of the N-dimensional space having        integer coordinates. This hyper-cubic constellation may, for        instance, be a generalization of a conventional regular QAM        constellation to N dimensions. However, other initial        constellations may be used, such as generalizations of circular        constellation to N dimensions, and so on.

Once the initial constellation is fixed, it may be subjected to arotation by applying the above defined multi-dimensional rotation matrix

-   -           to each of the initial constellation points so as to obtain a        rotated set of constellation points, i.e., a multi-dimensional        rotated constellation. The multi-dimensional rotated        constellation may be more favorable than the initial        constellation in terms of the degree of modulation diversity        provided, depending on the particular choice of the “rotation        factor” r. The “rotation factor” r, and therewith the rotated        constellation, may be varied, as described above, so as to        obtain a constellation that provides maximum modulation        diversity, or at least a certain minimum degree of modulation        diversity, as required by the specific application.

The present invention also provides a method and an apparatus forefficiently transmitting and receiving data over a plurality of fading(sub-) channels or slots on the basis of a modulation scheme thatemploys a multi-dimensional rotated constellation as obtained by theabove described method. The inventive method or apparatus may eitherperform the above described method in order to obtain the desiredmulti-dimensional rotated constellation, or use a set of predefined andprestored constellation points of the multi-dimensional rotatedconstellation that have been calculated using the above describedmethod. In the latter case, the inventive method or apparatus may accessa storage means, wherein information indicating the positions of atleast some of the constellation points is stored.

Another aspect of the present invention concerns the separation andmapping of the N dimensions of the N-dimensional rotated constellationso that they experience independent fading during transmission. This isa key aspect necessary for achieving the expected diversity performance.

Generally, this can be achieved by transmitting each of the N componentsof a constellation point of an N-dimensional rotated constellation overa different one of a plurality of transmission channels, provided thatfading of each of these transmission channels is uncorrelated withfading of any other one of the transmission channels. Here, the phrase“a different one of a plurality of transmission channels” may refer to adifferent one of a plurality of time slots, frequencies, transmitterantennas, or combinations thereof. In the context of orthogonalfrequency-division multiplexing (OFDM), the phrase “a different one of aplurality of transmission channels” may in particular refer to adifferent one of a plurality of active carriers, OFDM symbols, orcombinations thereof. In the context of a single carrier system, thephrase “a different one of a plurality of transmission channels” may inparticular refer to a different one of a plurality of symbols or timeslots.

Further signal processing is possible before transmission. The criticalaspect is that fading experienced by each of the N dimensions must bedifferent from, or ideally uncorrelated with, fading experienced by anyother one of the N dimensions.

The spreading of the N dimensions across different time slots,frequencies, and transmitter antennas can be achieved for examplethrough appropriate interleaving and mapping.

Another aspect of the present invention concerns the mapping of the Nreal dimensions of the N-dimensional rotated constellation to complexsymbols for transmission. Since fading of the in-phase component and thequadrature component of a given channel is typically identical, acomplex symbol may not be made up of two different components of thesame constellation point. Instead, the N components of a constellationpoint must be mapped to different complex symbols in order to ensure thedesired diversity.

The complex symbols generated in this manner are then spread in aconventional manner over the available time slots, frequencies, and/orantennas, e.g. through interleaving and mapping, so that fadingexperienced by each of the N dimensions is uncorrelated with fadingexperienced by any other one of the N dimensions.

The following describes an example flow of a method for generating amulti-dimensional constellation for a digital modulation scheme in datatransmission. This flow is achieved by, for example, a computer system.Each of the following steps is executed by a central processing unit(CPU).

(Step 1) A plurality of vectors of an N-dimensional vector space arereceived. Note, for example, the plurality of received vectors representa subset of

-   ^(N ,)    which is an N-dimensional integer lattice.

(Step 2) Sign values s_(i,j) of the sign matrix shown in the above Math.24 are determined, such that the N-dimensional rotation matrix

-       shown in the above Math. 20 is orthogonal.

(Step 3) A “rotation factor” r is selected as a real number between 0and 1. It should be noted that the “rotation factor” r, for example, isselected so that it maximizes the minimum number of different values inthe components of any two distinct multi-dimensional constellationpoints. However, the present invention is not limited to this.Alternatively, the “rotation factor” r may be selected so that itincreases a minimum number of different values in components of any twodistinct N-dimensional rotated constellation points relative to aminimum number of different values in components of any two distinctvectors received in Step 1.

(Step 4) Values of the parameters a and b are calculated by substitutingthe value of the “rotation factor” r, which has been selected in Step 3,into the above Math. 23.

(Step 5) The N-dimensional rotation matrix

-   -           is determined from the above Math. 20 by using (i) the sign        matrix

-       having the sign values s_(i,j) determined in Step 2, and (ii) the    values of the parameters a and b calculated in Step 4.

(Step 6) A constellation point of the N-dimensional rotatedconstellation is obtained by applying the N-dimensional rotation matrix

-       determined in Step 5 to the plurality of vectors of the    multi-dimensional vector space, which have been received in Step 1.

FIG. 12 is a block diagram of a transmission apparatus according to anembodiment of the present invention, which is similar to the one shownin FIG. 2. The elements that are the same as those described above areassigned the same reference numerals thereas, and a detailed explanationthereof is omitted.

The transmission apparatus of FIG. 12 differs from that of FIG. 2 inthat the rotated constellation demapper 230 is replaced with a rotatedconstellation demapper 1230. The rotated constellation demapper 1230performs processing on the basis of an N-dimensional rotatedconstellation that has a plurality of constellation points defined bypositions thereof within an N-dimensional space, the positions beingobtained by applying either the N-dimensional rotation matrix shown inthe above Math. 20, or an N-dimensional rotation matrix obtained bypermuting rows and/or columns of the N-dimensional rotation matrix shownin the above Math. 20, to a subset of

-   -   ^(N ,)        which is the N-dimensional integer lattice. To be more specific,        this processing is to map the output of the bit interleaver 220        to the rotated constellation.

FIG. 13 is a block diagram of a reception apparatus according to anembodiment of the present invention, which is similar to the one shownin FIG. 4. The elements that are the same as those described above areassigned the same reference numerals thereas, and a detailed explanationthereof is omitted.

The reception apparatus of FIG. 13 differs from that of FIG. 4 in thatthe rotated constellation demapper 450 is replaced with a rotatedconstellation demapper 1350. The rotated constellation demapper 1350performs processing on the basis of an N-dimensional rotatedconstellation that has a plurality of constellation points defined bypositions thereof within an N-dimensional space, the positions beingobtained by applying either the N-dimensional rotation matrix shown inthe above Math. 20, or an N-dimensional rotation matrix obtained bypermuting rows and/or columns of the N-dimensional rotation matrix shownin the above Math. 20, to a subset of

-   -   ^(N ,)        which is the N-dimensional integer lattice.

FIG. 14 shows an example hardware implementation for the rotatedconstellation demapper 1350 of FIG. 13 for a 16-QAM rotatedconstellation (N=2, B=2). The rotated constellation demapper 1350 ofFIG. 13 includes a rotated constellation mapper 1420, instead of therotated constellation mapper 720 shown in FIG. 7. The rotatedconstellation mapper 1420 maps the outputs b₁ to b₄ from the counter 710to an N-dimensional rotated constellation that has a plurality ofconstellation points defined by positions thereof within anN-dimensional space, the positions being obtained by applying either theN-dimensional rotation matrix shown in the above Math. 20, or anN-dimensional rotation matrix obtained by permuting rows and/or columnsof the N-dimensional rotation matrix shown in the above Math. 20, to asubset of

-   -   ^(N ,)        which is the N-dimensional integer lattice. Then, the rotated        constellation mapper 1420 outputs the resulting constellation        components s₁ to s₄ to the squared Euclidean distance calculator        730.

It should be noted that the structures of the transmission apparatus andthe reception apparatus are not limited to those described above. Forexample, the reception apparatus may have either one of the structuresshown in FIGS. 10 and 11. In this case, the rotated constellationdemapper 1010 or 720 performs processing on the basis of anN-dimensional rotated constellation that has a plurality ofconstellation points defined by positions thereof within anN-dimensional space, the positions being obtained by applying either theN-dimensional rotation matrix shown in the above Math. 20, or anN-dimensional rotation matrix obtained by permuting rows and/or columnsof the N-dimensional rotation matrix shown in the above Math. 20, to asubset of

-   -   ^(N ,)        which is the N-dimensional integer lattice.

The present invention relates to digital data communication and providesan efficient method for generating multi-dimensional constellations fordigital data modulation with a high degree of modulation diversity, amethod for transmitting and receiving data on the basis of suchconstellations, and a corresponding apparatus. This is achieved byconsidering only multi-dimensional rotation matrices with all elementson the diagonal having the same first absolute value and all otherelements having the same second absolute value. In this manner,multi-dimensional rotation matrices can be generated having a singleindependent parameter and a structure that is as regular as possible.The independent parameter can be configured in order to minimize theerror probability for various constellation sizes.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a communication apparatus thatperforms modulation/demodulation by using a constellation.

REFERENCE SIGNS LIST

210 FEC encoder

220 bit interleaver

1230 rotated constellation mapper

240 complex symbol mapper

250 symbol interleaver/mapper

260-1 to 260-M modulation chain

270-1 to 270-M transmitter antenna

410-1 to 410-M receiver antenna

420-1 to 420-M demodulation chain

430 symbol demapper/deinterleaver

440 complex symbol demapper

1350 rotated constellation demapper

460 bit deinterleaver

470 FEC decoder

The invention claimed is:
 1. A transmission apparatus for transmitting ablock of data over a plurality of transmission channels, thetransmission apparatus comprising: a modulator operable to generate oneof a plurality of rotated constellation points in accordance with theblock of data to be transmitted, each of the plurality of rotatedconstellation points having N components; a mapper operable to separateand map each of the N components of the generated rotated constellationpoint to a different one of a plurality of complex symbols; and atransmitter operable to transmit each of the complex symbols at adifferent time and/or at a different frequency, wherein the plurality ofrotated constellation points are obtained by applying an orthogonaltransformation to each of a plurality of initial constellation pointshaving N components so as to increase a reception robustness underfading conditions, N is a multiple of four, the orthogonaltransformation has one of (i) an N-by-N matrix representation withabsolute values of all elements on a main diagonal equal to a firstvalue, and with absolute values of all elements not on the main diagonalequal to a non-zero second value, and (ii) a matrix representationobtained by permuting rows and/or columns in an N-by-N matrixrepresentation with absolute values of all elements on a main diagonalequal to a first value, and with absolute values of all elements not onthe main diagonal equal to a non-zero second value, and when each of theN-by-N matrix representations is denoted by R, $R = \begin{pmatrix}{s_{1,1}a} & {s_{1,2}b} & \ldots & {s_{1,N}b} \\{s_{2,1}b} & {s_{2,2}a} & \; & \vdots \\\vdots & \; & \ddots & {s_{{N - 1},N}b} \\{s_{N,1}b} & \ldots & {s_{N,{N - 1}}b} & {s_{N,N}a}\end{pmatrix}$ a and b denote real parameters, each sign value s_(i,j)satisfiess_(i,j) ε{−1,+1}, and the real parameters a and b satisfya²+(N−1)b² =1.
 2. A reception apparatus for receiving a block of dataover a plurality of transmission channels, the reception apparatuscomprising: a receiver operable to receive a plurality of receivedcomplex symbols at a different time and/or at a different frequency, oneof a plurality of rotated constellation points being generated inaccordance with the block of data, each of the plurality of rotatedconstellation points having N components, each of the N components ofthe generated rotated constellation point being separated and mapped toa different one of a plurality of transmitted complex symbols, each ofthe plurality of the transmitted complex symbols being transmitted at adifferent time and/or at a different frequency, the plurality of thereceived complex signals being obtained by receiving the plurality ofthe transmitted complex signals; and a demodulator operable todemodulate N components obtained from the plurality of received complexsymbols based on a rotated constellation, wherein the plurality ofrotated constellation points are obtained by applying an orthogonaltransformation to each of a plurality of initial constellation pointshaving N components so as to increase a reception robustness underfading conditions, N is a multiple of four, and the orthogonaltransformation has one of (i) an N-by-N matrix representation withabsolute values of all elements on a main diagonal equal to a firstvalue, and with absolute values of all elements not on the main diagonalequal to a non-zero second value, and (ii) a matrix representationobtained by permuting rows and/or columns in an N-by-N matrixrepresentation with absolute values of all elements on a main diagonalequal to a first value, and with absolute values of all elements not onthe main diagonal equal to a non-zero second value, and when each of theN-by-N matrix representations is denoted by R, $R = \begin{pmatrix}{s_{1,1}a} & {s_{1,2}b} & \ldots & {s_{1,N}b} \\{s_{2,1}b} & {s_{2,2}a} & \; & \vdots \\\vdots & \; & \ddots & {s_{{N - 1},N}b} \\{s_{N,1}b} & \ldots & {s_{N,{N - 1}}b} & {s_{N,N}a}\end{pmatrix}$ a and b denote real parameters, each sign value s_(w)satisfiess_(i,j) ε{−1,+1}, and the real parameters a and b satisfya²+(N−1)b²=1.
 3. A transmission method for transmitting a block of dataover a plurality of transmission channels, the transmission methodcomprising the steps of: generating one of a plurality of rotatedconstellation points in accordance with the block of data to betransmitted, each of the plurality of rotated constellation pointshaving N components; separating and mapping each of the N components ofthe generated rotated constellation point to a different one of aplurality of complex symbols; and transmitting each of the complexsymbols at a different time and/or at a different frequency, wherein theplurality of rotated constellation points are obtained by applying anorthogonal transformation to each of a plurality of initialconstellation points that having N components so as to increase areception robustness under fading conditions, N is a multiple of four,and the orthogonal transformation has one of (i) an N-by-N matrixrepresentation with absolute values of all elements on a main diagonalequal to a first value, and with absolute values of all elements not onthe main diagonal equal to a non-zero second value, and (ii) a matrixrepresentation obtained by permuting rows and/or columns in an N-by-Nmatrix representation with absolute values of all elements on a maindiagonal equal to a first value, and with absolute values of allelements not on the main diagonal equal to a non-zero second value, andwhen each of the N-by-N matrix representations is denoted by R,$R = \begin{pmatrix}{s_{1,1}a} & {s_{1,2}b} & \ldots & {s_{1,N}b} \\{s_{2,1}b} & {s_{2,2}a} & \; & \vdots \\\vdots & \; & \ddots & {s_{{N - 1},N}b} \\{s_{N,1}b} & \ldots & {s_{N,{N - 1}}b} & {s_{N,N}a}\end{pmatrix}$ a and b denote real parameters, each sign value s_(i,j)satisfiess_(i,j)ε{−1,+1}, and the real parameters a and b satisfya²+(N−1)b²=1.
 4. A reception method for receiving a block of data over aplurality of transmission channels, the reception method comprising thesteps of: receiving a plurality of received complex symbols at adifferent time and/or at a different frequency, one of a plurality ofrotated constellation points being generated in accordance with theblock of data, each of the plurality of rotated constellation pointshaving N components, each of the N components of the generated rotatedconstellation point being separated and mapped to a different one of aplurality of transmitted complex symbols, each of the plurality of thetransmitted complex symbols being transmitted at a different time and/orat a different frequency, the plurality of the received complex signalsbeing obtained by receiving the plurality of the transmitted complexsignals; demodulating N components obtained from the plurality ofreceived complex symbols based on a rotated constellation, wherein theplurality of rotated constellation points are obtained by applying anorthogonal transformation to each of a plurality of initialconstellation points having N components so as to increase a robustnessunder fading conditions, N is a multiple of four, and the orthogonaltransformation has one of (i) an N-by-N matrix representation withabsolute values of all elements on a main diagonal equal to a firstvalue, and with absolute values of all elements not on the main diagonalequal to a non-zero second value, and (ii) a matrix representationobtained by permuting rows and/or columns in an N-by-N matrixrepresentation with absolute values of all elements on a main diagonalequal to a first value, and with absolute values of all elements not onthe main diagonal equal to a non-zero second value, and when each of theN-by-N matrix representations is denoted by R, $R = \begin{pmatrix}{s_{1,1}a} & {s_{1,2}b} & \ldots & {s_{1,N}b} \\{s_{2,1}b} & {s_{2,2}a} & \; & \vdots \\\vdots & \; & \ddots & {s_{{N - 1},N}b} \\{s_{N,1}b} & \ldots & {s_{N,{N - 1}}b} & {s_{N,N}a}\end{pmatrix}$ a and b denote real parameters, each sign value s_(i,j)satisfiess_(i,j)ε{−1,+1}, and the real parameters a and b satisfya²+(N−1)b²=1.
 5. A generation method for generating an N-dimensionalrotated constellation for a digital modulation scheme in a datacommunication system, the generation method comprising the steps of:receiving a plurality of vectors of an N-dimensional vector space; andobtaining constellation points of the N-dimensional rotatedconstellation by applying an orthogonal transformation to the pluralityof vectors received, each of the constellation points having Ncomponents, each of the N components being separated and mapped to adifferent one of a plurality of complex symbols, each of the pluralityof complex symbols being transmitted at a different time and/or at adifferent frequency, wherein the orthogonal transformation is adaptedfor increasing a minimum number of different values in components of anytwo distinct N-dimensional rotated constellation points relative to aminimum number of different values in components of any two distinctvectors received, and the orthogonal transformation has one of (i) anN-by-N matrix representation, N being a multiple of four, with absolutevalues of all elements on a main diagonal equal to a first value, andwith absolute values of all elements not on the main diagonal equal to anon-zero second value, and (ii) a matrix representation obtained bypermuting rows and/or columns in an N-by-N matrix representation withabsolute values of all elements on a main diagonal equal to a firstvalue, and with absolute values of all elements not on the main diagonalequal to a non-zero second value, and when each of the N-by-N matrixrepresentations is denoted by R, $R = \begin{pmatrix}{s_{1,1}a} & {s_{1,2}b} & \ldots & {s_{1,N}b} \\{s_{2,1}b} & {s_{2,2}a} & \; & \vdots \\\vdots & \; & \ddots & {s_{{N - 1},N}b} \\{s_{N,1}b} & \ldots & {s_{N,{N - 1}}b} & {s_{N,N}a}\end{pmatrix}$ a and b denote real parameters, each sign value s_(i,j)satisfiess_(i,j)ε{−1,+1}, and the real parameters a and b satisfya²+(N−1)b²=1.